1. Field of the Invention
The present invention generally relates to a system for defrosting a transparent plastic panel.
2. Description of Related Art
Plastic windows, such as polycarbonate windows, have recently become available as an alternative to glass windows. In vehicle applications, plastic windows may provide a significant reduction in weight, as well as, provide other attractive attributes. However, lower electrical conductivity has been exhibited by conductive pastes printed and cured on plastic substrates, when compared to sintered metallic pastes printed on glass substrates. In addition, lower thermal conductivity is also typically exhibited by plastics as compared to glass. As a result, heater grid functionality further suffers when long grid lines are required on a plastic panel, such as a rear window. As seen from the above, there is a need in the industry to enhance and optimize the amount of heat generated and dissipated during a defrosting cycle in order to provide acceptable heater grids for large windows.
Some defroster grid patterns work very well at voltages well below the battery voltage present in an automobile. For such systems, applying the battery voltage, (typically about 13 volts) may result in overheating of the grid, possible melting of the plastic panel or destruction of the defroster grid itself. As a result, the utilization of these defroster grids requires a high wattage resistor to be connected in electrical series with the defroster grid to step down the voltage. For example, if a defroster grid pattern has an optimum operation voltage of around seven volts, a resistor would be required that dissipates almost 89% of the power used by the defroster itself. Therefore, the defroster circuit in the vehicle would be required to handle almost twice the power actually used by the defrosting grid. In addition, the resistor itself would generate a large amount of heat that must be separately managed.
In view of the above, it is apparent that there exists a need for an improved system for defrosting plastic windows in vehicle applications.